Method for decoding digital data in a frequency hopping communication system

ABSTRACT

An improved method is provided for decoding data in a frequency hopping communications system. The method includes: monitoring transition points between data bits in a demodulated data stream, where the data bits are transmitted to a receiver over different transmission frequencies; determining a frequency over which data bits are transmitted to the receiver; determining a reliability metric for each frequency over which data bits were received, where the reliability metric is based on transition points of data bits transmitted over a given frequency; and performing a decoding operation using the reliability metric for each frequency over which data bits were received.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.11/377,491 filed on Mar. 16, 2006. The entire disclosure of the aboveapplication is incorporated herein by reference.

FIELD

The present disclosure relates generally to frequency hoppingcommunication systems and, more particularly, to an improved method fordecoding frequency hopped digital data using a forward error correctingscheme with erasure decoding capabilities.

BACKGROUND

In a frequency hopped waveform, it is possible to achieve modemacquisition in very high bit rate error conditions. In a jammingenvironment specifically, a modem can acquire with an on air bit errorrate sometimes greater than 25-30%. However, it is important in thisenvironment to provide a method of robust error correction in order toreliably decode a data stream at the receiver.

Forward error correction (FEC) is a method of controlling the receivederror rate of a user-provided data stream that is transmitted over anoisy channel. A forward error correction scheme will in generalaccomplish this by the generation and transmission of extra data alongwith the user data stream. This extra data provides the receiver with amore efficient way of determining what was actually transmitted acrossthe channel. The performance of some forward error correcting schemescan be enhanced by indicating to the decoder which portions of a receivedata stream are likely to be in error. The indicated portions arereferred to as erasures. A forward error correction scheme that can makeuse of this information is referred to as having erasure decodingcapabilities.

Accordingly, it is desirable to provide a method for decoding digitaldata using a forward error correcting scheme with erasure decodingcapabilities in the context of a frequency hopping communication system.The statements in this section merely provide background informationrelated to the present disclosure and may not constitute prior art.

SUMMARY

An improved method is provided for decoding data in a frequency hoppingcommunications system. The method includes: monitoring transition pointsbetween data bits in a demodulated data stream, where the data bits aretransmitted to a receiver over different transmission frequencies;determining a reliability metric for each frequency over which data bitswere transmitted, where the reliability metric is based on transitionpoints of data bits transmitted over a given frequency; and performing adecoding operation using the reliability metric for each frequency overwhich data bits were transmitted to the receiver.

Further areas of applicability will become apparent from the descriptionprovided herein. It should be understood that the description andspecific examples are intended for purposes of illustration only and arenot intended to limit the scope of the present disclosure.

DRAWINGS

FIG. 1 is a flowchart depicting an improved method for improved methodis provided for decoding data in a frequency hopping communicationssystem;

FIG. 2 is a diagram illustrating how data bit transitions can provide anindication as to the reliability of the frequency over which the databit was transmitted;

FIG. 3 is a block diagram illustrating how the improved decoding methodmay be integrated into a radio's receiver architecture; and

FIG. 4 is a flowchart further illustrating the improved decoding method.

The drawings described herein are for illustration purposes only and arenot intended to limit the scope of the present disclosure in any way.

DETAILED DESCRIPTION

Frequency hopping is a method of transmitting radio signals by rapidlytransmitting a carrier among many frequency channels. Briefly, atransmitter “hops” between available frequencies according to aspecified algorithm. The transmitter operates in synchronization with areceiver, which remains tuned to the same frequency as the transmitter.A short burst of data is transmitted on a particular carrier frequency.The transmitter then tunes to another frequency and transmits again.Thus, the receiver is capable of hopping its frequency several times asecond to follow the transmission frequency employed by the transmitter.

FIG. 1 depicts an improved method 10 for decoding digital datatransmitted in a frequency hopping communication system. Thecommunication system is assumed to employ a forward error correctingscheme with erasure capabilities. While the decoding methods of thisdisclosure are described in the context of a frequency hoppingcommunication system, they have application to other types of radiocommunication systems which transmit signals over multiple frequencies.

First, the reliability of data bits received at a receiver is evaluatedat 12 based on transition points between the data bits. A basebandsample stream must be sampled correctly with respect to its data bittransitions in order to have its modulated data correctly recovered.Thus, receiver's demodulator is aligned with the data bit transitions ofthe stream in a manner known in the art.

For a baseband sample stream, the transition points occur as signchanges and are commonly referred to as zero crossings. Given the knownbit rate for the sample stream, an expected transition point can bederived for each data bit in the sampled stream. These data bittransitions are then monitored and assessed to determine whether theyoccur within an expected transition region as shown in FIG. 2. Data bitsfalling within the expected transition region as indicated at 22 haveexperienced considerably less noise than data bits falling out side ofthe expected transition region as indicated as 24. Thus, data bittransitions can provide an indication as to the reliability of thefrequency over which the data bit was transmitted to the receiver.

Based on data bit transitions, reliability metrics may be determined at16 for each frequency over which data bits were received at thereceiver. In an exemplary embodiment, the reliability metric is derivedfrom a comparison between the number of transition points which fallwithin the expected transition region and the total number of transitionpoints seen. In other words, the reliability metric is defined as aratio of the number of zero crossing that occur in the expectedtransition region to the total number of zero crossings that occur bothinside and outside of the expected transition region. Other reliabilitymetrics based on data bit transitions are also contemplated by thisdisclosure. Likewise, reliability metrics may be derived from othersignal parameters. For example, a reliability metric may be based on aratio of any variation in the RF signal envelope to a mean value for theRF signal envelope

In order to tune an applicable frequency, the demodulator in a frequencyhopped system knows the hopping sequence. Accordingly, the demodulatorknows the frequency over which each data bit was transmitted to thereceiver. For each hops' worth of data, a reliability ratio is computedfrom the data bits that were transmitted over the given frequency.Alternatively, the reliability ratio may be computed from data bitstransmitted over a given frequency but sent over multiple hops.

For illustration purposes, a simple example is further described below.Assume five codewords each comprised of five bits are transmitted overfive different frequencies to the receiver. These codewords are noted asfollows:

CW₁=b₁₁b₁₂b₁₃b₁₄b₁₅

CW₂=b₂₁b₂₂b₂₃b₂₄b₂₅

CW₃=b₃₁b₃₂b₃₃b₃₄b₃₅

CW₄=b₄₁b₄₂b₄₃b₄₄b₄₅

CW₅=b₅₁b₅₂b₅₃b₅₄b₅₅

Prior to being transmitted, these codewords may be interleaved such thatthe first bit of each codeword is assigned to a first interleavedsequence, the second bit of each codeword is assigned to a secondinterleaved sequence, and so on. The resulting interleaved sequences areas follows:

IL1=b₁₁b₂₁b₃₁b₄₁b₅₁

IL2=b₁₂b₂₂b₃₂b₄₂b₅₂

IL3=b₁₃b₂₃b₃₃b₄₃b₅₃

IL4=b₁₄b₂₄b₃₄b₄₄b₅₄

IL5=b₁₅b₂₅b₃₅b₄₅b₅₅

The interleaved sequences are then assigned a frequency fortransmission.

Upon receipt of each bit sequence at the receiver, a reliability ratiomay be computed from the bits contained in the sequence. For instance,if three of the bits in the first interleaved sequence fell within theexpected transition regions, and two of the bits fell outside of theexpected transition region, the reliability ratio for the assignedfrequency is computed as ⅗=0.6. In the second interleaved sequence,perhaps only a single bit fell outside of the expected transitionregion. In this case, the reliability ratio for this assigned frequencyis computed as ⅘=0.8. A reliability ratio for each frequency may becomputed in a similar manner.

On a high signal to noise (i.e., low noise) frequency channel, thisratio will be close to unity. As the noise level increases, the ratiowill also decease accordingly as more and more bit transitions occuroutside the expected region. Thus, this ratio provides an indication asto the reliability of the frequency. In an exemplary embodiment, thisratio may be compared to an empirically derived threshold value. Whenthe ratio for a given frequency greater than or equal to the threshold,the frequency is classified as reliable. When the ratio for the givenfrequency is lower than the threshold, the frequency is classified asunreliable. Alternatively, the ratio may be compared to multiplethresholds to determine different degrees of reliability. Thisreliability information is subsequently used when performing an decodingoperation as indicated at 18. It is readily understood that thethreshold may be set and adjusted based on system performanceobjectives.

FIG. 3 further illustrates how this decoding method may be integratedinto a radio's receiver architecture. In this exemplary embodiment, theradio receiver is generally comprised of a demodulator 32, ade-interleaver 36, and a decoder 38. While operation of these basiccomponents is further described below, it is understood that other knownradio receiver components may be needed for overall operation of thesystem.

In operation, a received data signal is first input to the demodulator32 which in turn demodulates the signal in a manner known in the art.The signal output from the demodulator 32 is commonly referred to as thebaseband sample stream.

The baseband sample stream is then input to a reliability metricgenerator 34. The reliability metric generator 34 is operable todetermine reliability information for each frequency over which databits were transmitted to the receiver. More specifically, thereliability metric generator 34 computes a reliability ratio for eachfrequency and compares the ratio to a threshold in the manner describedabove. This reliability information is then associated with each databit in the baseband sample stream. In an exemplary embodiment, a binaryindicator bit 35 is associated with each data bit. When a givenfrequency is deemed reliable, the reliability indicator 35 is set toone; whereas, when the given frequency is deemed unreliable, thereliability indicator 35 is set to zero. In an alternative embodiment,the reliability indicator may be comprised of two or more bits torepresent different degrees of reliability information. This reliabilityindicator is commonly referred to as an erasure.

Continuing with the example described above, the baseband sample streammay be comprised of the interleaved sequences IL1+IL2+IL3+IL4+IL5, wherethe each sequence was transmitted over a different frequency. Given athreshold of 0.7, the frequency assigned to the first interleavedsequence is deemed unreliable (i.e., 0.6<0.7). In this case, areliability indicator bit 35 of zero is associated with each bit in thesequence, such that the bit stream output by the reliability metricgenerator is as follows: (b₁₁,0), (b₂₁,0), (b₃₁,0), (b₄₁,0), (b₅₁,0). Incontrast, the frequency assigned to the second interleaved sequence isdeemed reliable (i.e., 0.83>0.7). A reliability indicator bit 35 of oneis associated with each bit in the sequence as follows: (b₁₂,1),(b₂₂,1), (b₃₂,1), (b₄₂,1), (b₅₂,1). Thus, the data bits of the basebandsample stream are reformulated into bit pairs.

Next, the reformulated bit pairs are input to the de-interleaver 36. Thede-interleaver 36 is adapted to account for the additional reliabilitybit associated with each incoming data bit, but otherwise operates inthe manner known in the art to re-order the bit pairs into a sequence ofcodewords. In the ongoing example, the re-ordered bit pairs appear as:(b₁₁,0), (b₁₂,1), (b₁₃,1) . . . .

Lastly, the stream of re-ordered bit pairs is input into a decoder whichemploys a forward error correcting scheme with erasure capabilities. Inthe case of a majority logic decoder, the method of applying thereliability indicator is easily understood. For each codeword, thedecoder determines the number of bits having a value of one and thenumber of bits having a value of zero. Bits indicated as beingtransmitted over an unreliable frequency are ignored. If there are moreones than zeroes in a given codeword, the decoder outputs a value of onefor the codeword. If there are more zeroes than ones in the codeword,the decoder outputs a value of zero for the codeword. For a tie, thedecoder may make an arbitrary decision. Within the scope of thisdisclosure, it is readily understood that the re-ordered bit pairs maybe input to decoder which employ other decoding schemes (e.g.,Reed-Solomon forward error correction scheme with erasure capabilities).In this way, the decoding accuracy of the data is improved in afrequency hopped communication system.

The improved decoding method described above is illustrated further inFIG. 4. This improved method is particularly suited for militaryapplications such as the Single Channel Ground-Airborne Radio System(SINCGARS). Further details regarding this radio system may be found inU.S. Pat. Nos. 6,018,543; 6,052,406; and 6,078,612 which areincorporated herein by reference. Nonetheless, the above description ismerely exemplary in nature and is not intended to limit the presentdisclosure, application, or uses

1. An improved method for decoding data in a frequency hoppingcommunications system, comprising: receiving an incoming radio signal bya radio and demodulating the incoming radio signal to form a demodulateddata system; monitoring transition points between data bits in ademodulated data stream to determine whether the transition pointsbetween the data bits fall within an expected transition region, wherethe data bits are transmitted to a receiver over different transmissionfrequencies; determining, for each of the different transmissionfrequencies, a reliability metric based on transition points of databits transmitted over the respective transmission frequency, thereliability metric having one of a first value indicating the respectivetransmission frequency is reliable or a second value indicating therespective transmission frequency is unreliable; assigning a distinctreliability indicator to each data bit in the demodulated data stream,where the reliability indicator for a given data bit is the same as thereliability metric for the frequency over which the given data bit wastransmitted to the receiver; and performing a decoding operation on thedata bits of the data stream, the decoding operation using thereliability indicator assigned to respective data bits in the datastream.
 2. The method of claim 1 wherein determining a reliabilitymetric further comprises assessing transition points of data bitstransmitted during a single hop on a given frequency.
 3. The method ofclaim 1 wherein determining a reliability metric further comprisesassessing whether the bit transitions are within an expected transitionregion as derived from a known bit rate of the data stream.
 4. Themethod of claim 1 wherein assessing the bit transitions furthercomprises: computing a ratio between a number of bit transitions whichfall inside of the expected transition region and a total number of bittransitions which fall both inside and outside the expected transitionregion; classifying a given frequency over which data bits were receivedas reliable when the ratio for the given frequency is greater than orequal to a predefined threshold; and classifying a given frequency overwhich data bits were received as unreliable when the ratio for the givenfrequency is less than the predefined threshold.
 5. The method of claim1 wherein the reliability metric is further defined as a binaryindicator which is set to one when the frequency is deemed reliable andset to zero when the frequency is deemed unreliable.
 6. An improvedmethod for decoding data in a frequency hopping communications system,comprising: receiving an incoming radio signal by a radio anddemodulating the incoming radio signal to form a demodulated datastream; monitoring transition points between data bits in a demodulateddata stream to determine whether the transition points between the databits fall within an expected transition region, where the data bits aretransmitted to a receiver over different transmission frequencies;determining, for each of different transmission frequencies, areliability metric based on the transition points of data bitstransmitted over the respective transmission frequency, the reliabilitymetric having one of a first value indicating the respectivetransmission frequency is reliable or a second value indicating therespective transmission frequency is unreliable; assigning a distinctreliability indicator to each different data bit in the demodulated datastream, where the reliability metric for data bits transmitted over agiven frequency is generated from the reliability indicator assigned toeach data bit transmitted over the given frequency and is distinct fromthe reliability metric for data bits transmitted over frequencies otherthan the given frequency; and performing a decoding operation on thedata bits of the data stream, the decoding operation using thereliability indicator assigned to respective data bits in the datastream.
 7. The method of claim 6 further comprises demodulating anincoming radio signal to form demodulated data stream.
 8. The method ofclaim 6 wherein determining a reliability metric further comprisesassessing transition points of data bits transmitted during a single hopon a given frequency.
 9. The method of claim 6 wherein determining areliability metric further comprises assessing whether the bittransitions are within an expected transition region as derived from aknown bit rate of the data stream.
 10. The method of claim 6 whereinassessing the bit transitions further comprises: computing a ratiobetween a number of bit transitions which fall inside of the expectedtransition region and a total number of bit transitions which fall bothinside and outside the expected transition region; classifying a givenfrequency over which data bits were received as reliable when the ratiofor the given frequency is greater than or equal to a predefinedthreshold; and classifying a given frequency over which data bits werereceived as unreliable when the ratio for the given frequency is lessthan the predefined threshold.
 11. The method of claim 6 wherein thereliability metric is further defined as a binary indicator which is setto one when the frequency is deemed reliable and set to zero when thefrequency is deemed unreliable.
 12. A receiver in a frequency hoppingcommunication system, comprising: a demodulator adapted to receive anincoming data signal and operable to demodulate the data signal to forma baseband data stream; a reliability metric generator adapted toreceive the baseband data stream from the demodulator and operable tocompute a reliability metric based on transition points of data bitstransmitted over the respective transmission frequency, the reliabilitymetric having one of a first value indicating the respectivetransmission frequency is reliable or a second value indicating therespective transmission frequency is unreliable, the reliability metricgenerator further operable to output a bit stream having a reliabilityindicator assigned to each data bit based on the frequency over whichthe data bit was transmitted to the receiver, where the reliabilitymetric for data bits transmitted over a given frequency is generatedfrom the reliability indicator assigned to each data bit transmittedover the given frequency and is distinct from the reliability metric fordata bits transmitted over frequencies other than the given frequency;and a decoder adapted to receive the bit stream from the reliabilitymetric generator and operable to decode the bit stream using thereliability indicator assigned to each data bit.
 13. The receiver ofclaim 12 further comprises a de-interleaver adapted to receive the bitstream from the reliability metric generator and operable to passthrough the reliability indicator assigned to each data bit to thedecoder.
 14. The receiver of claim 12 wherein the reliability metricgenerator computes the reliability indicator by assessing whether bittransitions transmitted over a given frequency are within an expectedtransition region.
 15. The receiver of claim 14 wherein the reliabilitymetric generator further comprises computing a ratio between a number ofbit transitions which fall inside the expected transition region and atotal number of bit transitions falling both inside and outside of theexpected transition regions and classifying the given frequency asreliable when the ratio for the given frequency is greater than apredefined threshold.